The present invention relates to an improved method for easily measuring the light transmittance of an optical lens, such as a spectacle lens or an eyeglass lens, and an apparatus therefor. In the context of this disclosure, spectacle lens shall include optical lenses used to make spectacles, eyeglasses, monocles, and other optical lenses worn by a person to correct visual acuity. In addition, the present invention can be utilized to determine the light transmittance of an optical lens that is neither worn by a person nor used to correct visual acuity. Also, in the present disclosure, the terms xe2x80x9ctransmittancexe2x80x9d and xe2x80x9ctransmissionxe2x80x9d are considered synonymous and equivalent terms and are used interchangeably.
In spectacle lens stores (e.g., eyeglass stores and optometrist offices), it is occasionally required that the light transmittance of a spectacle lens with respect to ultraviolet light or visible light be examined and determined as one of several optical lens properties evaluated when choosing a spectacle lens for a customer. Typically, one favored method for measuring the light transmittance of a spectacle lens involves using an integrating sphere in an optical measuring system for detecting light accurately without causing errors in the measured value, even when the measured light is diffused or converged by the lens for examination (also referred to as the examined lens). However, the integrating sphere is expensive thereby making it difficult and costly to distribute integrating spheres widely among spectacle lens stores. Therefore, apparatuses for easily measuring light transmittance of a spectacle lens without using an integrating sphere have been proposed. For example, one apparatus and corresponding method for correcting the value of light transmittance in accordance with the dioptric power of the lens for examination (i.e., examined lens) is disclosed in paragraph [0027] of Japanese Patent Application Laid-Open No. Heisei 11(1999)-211617.
FIG. 7 herein shows a diagram describing the principle of the method disclosed in Japanese Patent Application Laid-Open No. Heisei 11(1999)-211617, mentioned above. As shown in FIG. 7, rays of light to be measured L emitted from a light source 1 are arranged into parallel rays by a convex lens 2, then pass through a lens being examined 3 and an interference filter 4 before being detected by a light receiving element 5. The amount of light transmitted through examined lens 3, being light transmission of the light rays having a wavelength in the range narrowed by the interference filter 4, can be easily obtained by calculation from the ratio of the intensity of the light from light source 1 detected by the light receiving element 5 in the absence of the lens for examination 3 to the intensity of light detected from light source 1 by the light receiving element 5 when the lens for examination 3 is placed between the light source 1 and the light receiving element 5. With this method, the light receiving element 5 can achieve a relatively good measurement accuracy with respect to the transmitted light because the light rays from source 1 are arranged as close to parallel as possible before reaching the light receiving element 5. Therefore, the light transmittance can be measured with a relatively good accuracy.
In another method as shown in FIG. 8 herein, the convex lens 2 shown in FIG. 7 is removed and the bundle of rays L originating from light source 1 are narrowed by a necessary amount by a pinhole in a pinhole element 7 before the rays L are sent to the lens for examination 3. In other words, the light rays L passing through the pinhole are sufficiently close to parallel so as to effect a reasonably accurate measurement of the light transmittance of the examined lens 3. In still another method as shown in FIG. 9, a convex lens 6 is disposed between the interference filter 4 and the light receiving element 5 in the optical system shown in FIG. 8 and the rays L that passed through the lens for examination 3 converge and are directed into the light receiving element 5.
However, the above methods for measuring light transmittance of an examined lens have a drawback in that the error in the light transmittance measurement increases as the dioptric power of the lens being examined increases. The present inventors have found that this increase in the measurement error of the light transmittance for an examined lens arises due to the following reason. When lenses have different dioptric powers, generally the curvatures of the surfaces of the lens at the front side and at the back side are different among the lenses. In other words, a spectacle lens having a first dioptric power will have a different front side and back side curvature than another spectacle lens having a different dioptric power. In accordance with the conventional light transmittance measuring methods, the sectional area of the bundle of light rays used for measuring the light transmittance of the lens that originate from source 1 and that pass through the examined lens 3 is relatively great. At least, the cross sectional area of the bundle of light rays is large enough to create a significant measuring error because each individual light ray strikes a different portion of the lens. Therefore, the curvature of the lens surface upon which a light ray impacts is significantly different depending on the position within the section of the bundle of rays, and this results in a refractive effect on each ray that is different depending on the position of the ray in the bundle of light rays. In addition, the thickness of the examined lens is also significantly different depending on the position of the ray within the bundle of light rays. Moreover, the above described effects are different between lenses having different dioptric powers. As the result, the area of the light ray bundle, the position on the lens upon which each light ray impacts, the direction and the amount of the incident rays of the light being used for measurement create different optical transmission effects depending on the dioptric power of the lens being examined within the light detection area of the light detector. It is believed that an error in the measurement of the light transmittance of the examined lens arises due to these effects.
Other factors magnify the light transmittance measuring error created by these optical effects. For example, when a light detector such as a photodiode is used, the sensitivity of light detection on the surface of the light detector is not always uniform along the entire surface of the light detector. In other words, the light detecting sensitivity of the photodiode varies due to differences in the light detecting sensitivity between different positions on the surface of the light detector. Therefore, should the position of the incident light ray change between measurements taken with the lens present and without the lens present, an additional error arises in the measurement of the light transmittance because the incident light ray will shift slightly due to the refractive effect of the examined lens which will cause the incident light to strike a different portion of the light detector and this different portion may have a different light detecting sensitivity. In accordance with the above conventional methods for measuring light transmittance, the sectional area and the incident direction of the bundle of rays used to measure light transmittance tend to effect the measured value of the light transmittance and this effect on the value of the measured light transmittance varies to a great degree depending on the dioptric power of the lens being examined. In addition, the position of impact of the incident light used to measure transmittance on the light detector changes to a great degree depending on the dioptric power of the lens for examination and an additional error is believed to arise from this effect as well.
Another factor affecting the light detector, such as a photodiode as described above, is that the sensitivity of light detection changes considerably depending on the angle of the incident light being measured with respect to the surface of the light detector. In other words, the actual sensitivity varies depending on the incident angle of the light rays. In the above conventional methods for measuring the light transmittance, each ray that is part of the bundle of light rays being measured passes through and is refracted by the lens undergoing examination at one of the lens""s surface positions having a curved shape. Moreover, the degree of the refraction of each ray varies depending on the dioptric power of the lens being examined. As a result, the incident angle of each ray of incident light being measured has a different incident angle with respect to the light detector than the other incident light rays and yet another error is believed to arise from this factor.
Another factor to consider is that the total thickness of the lens undergoing examination, through which the bundle of incident light rays being measured will pass, is different depending on the dioptric power of the lens for examination. In other words, the dioptric power of a lens is dependent upon the thickness of the lens. Therefore, when the bundle of incident light rays to be measured has a relatively great sectional area and the dioptric power of the examined lens is great, then the amount of light attenuation cannot be neglected and an error is believed to arise in the measurement of light transmittance from this effect as well.
In view of these various sources of measuring errors, it is known that when using the apparatus for measuring spectacle lens light transmittance as disclosed in Japanese Patent Application Laid-Open No. Heisei 11(1999)-211617, the dioptric power of the examined lens must be known in advance in order to determine the light transmittance of the examined lens, which adversely affects the ease of the light transmittance measurement significantly.
The present invention endeavors to overcome the above drawbacks of the prior art light transmittance measuring systems and has an object of providing a method for measuring light transmittance which provides an inexpensive and easy light transmittance measurement of an optical lens having refractive (i.e., dioptric) power, thereby providing a method with excellent accuracy and an apparatus therefor.
As the means for solving the above problems and drawbacks of the prior art methods, the present inventors have developed the following invention. The method according to the invention includes a method for measuring light transmittance which comprises obtaining the light transmittance of a lens undergoing examination from a value corresponding to a ratio between an intensity of measured light detected by a means for detecting light when the examined lens is placed in a path of incident light to be measured that is emitted from a light source, passes through the examined lens and reaches a means for detecting light and an intensity of measured light detected by the means for detecting light when no examined lens is placed in the path of the measured light so that the measured light does not pass through the lens undergoing examination, wherein when the measured light passes through the lens undergoing examination, the measured light is in a condition such that rays of the measured light converge at or in a vicinity of a position where the lens undergoing examination is disposed.
The apparatus according to the invention includes an apparatus for measuring light transmittance which comprises: (a) a light source emitting incident light to be measured, (b) a light detector for detecting the light to be measured, (c) an apparatus for holding a lens undergoing examination, wherein the apparatus for holding a lens is disposed between the light source and the light detector and can hold or release the lens undergoing examination as desired, and (d) a first convergence lens through which rays of the light to be measured converge at, or in a vicinity of, the position where the lens undergoing examination is disposed.